A problem with pumps and motors of the above description is that there is a conflict between the need to prevent cavitation and excessive pressurization of cylinder walls in the rotor and the desire to provide a constant clamping force between the rotor and port plate. Cavitation results from implosions of gases entrained in the fluid which is in the cylinders. These implosions occur as a consequence of decompression of a cylinder after it has departed from registration with an arcuate port of the port plate (the lowpressure port in the case of a pump or the high-pressure port in the case of a motor). The greater the arc over which the cylinder travels under this condition, the greater is the possibility of cavitation. Excessive pressurization occurs when the cylinder travels over too large a precompression zone before fluid is released from the cylinder. The cavitation and/or excessive pressurization problems may be solved by extending the angles subtended by the arcuate ports so that the foresaid arc is sufficiently small. A known approach toward solving the excessive pressurization problem is to provide in the port plate a hole through which fluid is transferred to the high-pressure arcuate port when the pressure in the cylinder reaches the pressure in the highpressure port (see, e.g. U.S. Pat. No. 4,540,345 Frazer). However, when design considerations dictate the use of a rotor having an odd number of cylinders (which is ordinarily the case), these solutions restrict the practicable geometry between the ports of the rotor and port plate such that the pump or motor must be designed to operate with a fluctuating number of high-pressure cylinders if both cavitation and excessive pressurization are to be prevented. Fluctuation in the number of high-pressure cylinders is accompanied by fluctuation in forward thrust load on the rotor, and by fluctuation in clamping force between the rotor and the port plate. Fluctuation in clamping force can be expected to result in uneven wearing of the interfacing surfaces of the rotor and the port plate, and in metering inefficiency resulting from leakage to case pressure (which in turn may impose practical limitations on operating speed). Fluctuation in thrust load on the rotor can be expected to result in accelerated or less uniform wearing of piston shoes and thrust bearings. Past attempts at alleviating these effects have focused on the use of timing ports in fluid communication with auxiliary hold-up pistons which provide supplemental clamping force when there is a higher number of high-pressure cylinders (see, e.g. U.S. Pat. No. 3,037,489 Douglas). That approach, which is compensatory rather than remedial in nature, provides only a partial solution and creates the further problem of increased noise resulting from periodic occlusion of fluid communication to the auxiliary hold-up pistons.
Accordingly, an objective of this invention is to provide hydraulic motors and pumps which reduce or prevent cavitation and excessive pressurization while simultaneously providing a constant or substantially constant clamping force between rotor and port plate.
Another objective of this invention is to provide such motors or pumps that do not require the use of auxiliary hold-up pistons.
A further objective of this invention is to provide such motors and pumps that can be operated at higher speeds.
A still further objective of this invention is to provide such motors or pumps that operate with a substantially constant thrust load on the rotor.
These and further objectives and advantages of the invention will be apparent from the following description which includes the appended claims and accompanying drawings.